1. Field of the Invention
The present invention relates generally to multiple power converters used in conjunction, and relates more particularly to synchronization of power converters used in conjunction with interleaved phases.
2. Description of Related Art
Performance improvements in interleaved, multiphase power supplies result from advantages such as reduced input current ripple, reduced peak output current and higher frequency output ripple current. The higher frequency output ripple current permits easier filtering of the output ripple current to remove the ripple. Multiple interleaved phases in switching power supplies also tends to improve power conversion efficiency. A particular type of multiphase switching power supply has a variable switching frequency to obtain desired power supply output characteristics.
A variable frequency switching power supply may operate in various modes at various times, depending upon desired characteristics. For example, a switching power supply may operate in continuous, discontinuous or transition mode, each of which have various advantages. For example, a switching power supply may have an inductor that is supplied with current for a given interval and permitted to discharge to a certain extent. Such a switching power supply operating in a continuous mode permits an inductor to discharge to a point where the inductor current is still positive, or above zero, before charging the inductor again. A discontinuous mode switching power supply permits the current in the inductor to drop and remain at zero for a finite time before charging the output inductor again in a subsequent switching cycle. A transition mode switching power supply permits the inductor to discharge to zero current, at which point a new charging cycle begins, so that the inductor current is prevented from becoming negative or remaining zero.
One advantage to transition mode operation is the potential for zero voltage and/or zero current switching in the power supply. Zero voltage switching and zero current switching permits switching losses to be reduced, which can be especially advantageous at high frequencies.
Another advantage to transition mode operation is that it provides a simple way to maintain a desired power factor for a power converter. A typical transition mode configuration for a power converter permits the current in the inductor to achieve a peak value that is proportional to the input voltage. The momentary average of the current through the inductor is proportional to the instantaneous value of the input voltage, which permits the power converter to draw power from an input source at unity power factor. It is desirable to maintain the power factor as close as possible to unity, so that the power converter appears as a purely resistive load on the input power line. Factors that contribute to improving the power factor include maintaining input voltage in phase with input current, and maintaining the input current as a sinusoid when the input voltage is a sinusoid. Transition mode operation tends to help support realization of a good power factor in a variable frequency switching power supply.
A variable frequency transition mode power converter can be viewed as a free running oscillator with the frequency being controlled by the amplitude of the inductor current. Two or more transition mode power converters may be paralleled to produce multiple phases and obtain the advantages discussed above. Due to the variable frequency nature of the power supply switching, it can be challenging to synchronize the various phases to obtain one or more of the above-described advantages, especially as frequency changes to deliver desired output power characteristics. One variable in the synchronization of the phases is the number of phases that are interleaved or combined. For example, if a multiphase interleaved power supply has two phases, the phase angle difference of the waveforms in each phase should be 180°. For a three-phase interleaved power supply, the phase angle difference for the waveforms in each phase should be maintained at 120°. In general, the phase angle separation is equal to 360°/N, where N is the number of phases in the interleaved power supply.
One way to correct for mismatch in phase separation is to employ a Phase Locked Loop (PLL) to maintain an appropriate phase angle separation. Such a concept is illustrated in U.S. Pat. No. 5,793,191, where a slave stage of a power converter is maintained 180° out of phase with a master power converter stage. This arrangement calls for special purpose components that can add to power converter cost, complexity and size. One drawback to this approach is the challenge of acquiring and maintaining a phase lock over a wide range of conditions. The PLL capture range must encompass the difference between the free-running frequencies of the master and slave(s), which may prove difficult or costly in practice. For example, if a change in load demand causes switching frequency to change rapidly, a large momentary frequency error may result, which can cause loss of phase lock.